Not Applicable.
1. Field of the Invention
This invention relates generally to electrically powered fail-safe actuators for use in a variety of rotary control devices, such as valves and dampers.
2. Description of Related Art
Electric actuators can be used in industrial applications to control full clockwise and counterclockwise positioning, and usually strive for the following characteristics:
high torque per volume;
reliability;
improved control of the speed of the device; and
reduced energy consumption.
Current technology aimed at providing an actuator with these features is described below. All current technology lacks critical features as outlined below that are provided by the instant invention and described in this application.
Fail-safe operation in an actuator is activated when power loss or other external failure condition causes the actuator, without benefit of external electric power, to move the valve to a pre-determined position. To achieve adequate torque, this requirement on an actuator usually necessitates an increase in volume, thus reducing the torque/volume ratio. Fail-safe actuation has been approached in several ways. Some approaches involve an energy storage means such as a spring that is used to move the valve or damper to a certain position when a prespecified condition or set of conditions occurs. U.S. Pat. No. RE30,135 discloses an electric fail-safe actuator that employs a spring which is wound to store energy during operation of the electric drive motor, and an electric clutch operable to disengage the drive motor from the actuator output shaft in response to loss of power from the electrical supply whereby the spring drives the valve in the opposite direction. This device depends on an electric clutch means for switching to power-fail mode. U.S. Pat. No. 5,915,668 discloses a valve actuating apparatus including an actuator having a spring connected to the valve control arm or the clutch assembly for normally biasing the clutch assembly to a first position along the guide member such that the valve control arm is in the closed position. Upon interruption of power, the spring forces the engagement member of the clutch assembly out of engagement with the detent in the guide member and the spring forces the clutch assembly to slide back along the guide member to the first position and rotates the valve control arm to the closed position. In this case, the power-fail spring is forcing the clutch assembly, not the valve control shaft.
U.S. Pat. No. 4,595,081 discloses an electric motor that rotates a valve shaft one direction and a spring which is wound during driving of the shaft by the motor. The motor drives the output shaft and winds the return spring by way of a speed-reducing, torque amplifying gear train. To enable the use of a lighter return spring and the use of a gear train effecting greater torque amplification from the motor to the output shaft, intermediate gears in the drive train apply winding torque to the return spring differentially of the drive torque applied to the output shaft. This device requires intermediate gears and the time differential to switch between the gears. U.S. Pat. No. 5,662,542 discloses an actuating drive with a spring return feature including an electric drive, a reduction gearing having a return spring tensionable by the actuating movement and serving for the spring return movement, a clutch between the electric drive and the reduction gearing and a centrifugal brake device actuated during the spring return movement.
Devices that could effect fail-safe actuation without a spring rely on bias force and clutch mechanisms. For example, U.S. Pat. No. 5,988,319 discloses an apparatus for effecting actuation of a device having a home position and a set position. The apparatus returns the device to the home position upon loss of power to the apparatus. In this invention, a bias member having a cocking mechanism and a release mechanism is disclosed as the preferred embodiment. The bias member provides a bias force to a bias shaft when the cocking mechanism is cocked and the release mechanism is released; the release mechanism is released when power is lost to the actuator. U.S. Pat. No. 4,533,114 discloses an actuator for a rotary valve including a fail-safe mechanism for automatically opening or closing the valve upon the occurrence of a predetermined condition, and for allowing manual operation of the valve. A biased clutch activates the fail-safe mechanism and also couples a worm-gear drive for effecting the manual operation of the valve.
Of foremost concern in actuator design is reducing the physical size of an actuator while maintaining or increasing its torque output. This can be accomplished by novel design and engineering of the component parts of the actuator. One critical component of the electric actuator is the clutching-declutching mechanism, the size and efficiency of which is crucial to actuator torque/volume output. Current technology for clutching mechanisms comprises intermediate gears to effectuate continuous operation between gear changes. U.S. Pat. No. 5,490,433 discloses a transmission subunit with an intermediate shaft having continuous gears of progressive pitch diameters (ramp gears) interposed between pairs of conventional gears. The geometry of the continuous gears permits input gears, output gears, and/or idler gears to freely and independently slide longitudinally the length of the intermediate shaft without disengaging. Helical or spur cut gears can be used throughout. During shifting, an idler quickly passes from a conventional gear to an intermediate, continuous gear where it changes speed ratio progressively until the new ratio is achieved. At this point the idler quickly moves on to the next conventional gear to complete the shift cycle. An automatic locking mechanism assures precise, fixed alignment. This invention requires the presence of ramp gears to implement shift continuity, a requirement that adds volume and reduces torque output because of idle time during gear changes. In addition, torque output is further reduced by clutch pin friction and pin alignment delay.
Reliability in electric actuators has to do with downtime due to failure which is, in part, determined by the actuator""s ease of maintainability. Easy to maintain actuators reduce net downtime and thus increase reliability. Reducing actuator complexity is one way to reduce downtime. Highly complex actuators interweave sequencing and operational activities and drive them with the same motor. With these types of devices, isolating a failure, either while testing the device or during operations, can be difficult due to the complex operational sequences required to accomplish actuation. In addition, maintaining such a device can be more complex. Dividing functionality to simplify individual sequences is one way to reduce complexity. The current technology usually comprises one motor that drives both the sequencing of operations in the actuator as well as the output of the actuator. For example, U.S. Pat. No. 5,195,721 discloses a fail safe valve actuator that is powered by an electric motor. A valve stem with a helical groove is moved in one direction by a ball nut rotated by the electric motor to move a valve member to its operating position. The valve member is held in operating position by a solenoid. When power fails, a spring moves the valve stem in the other direction to move the valve member to its fail safe position. In this device, a centrifugal brake is required to limit torque to protect the electric motor from the high torque created when the valve stem abruptly stops moving when the valve member reaches its operating position and when the power fails and the valve stem is moved rapidly to its fail safe position by the spring. Also, energy to power the fail-safe mode is usually stored during normal operation and released upon detection of the condition. The current technology in most fail-safe mechanisms for electric valve actuators requires that the stored energy be maintained by a constant power supply, for example, U.S. Pat. No. 5,195,721. In this invention, a compression spring is used, versus a power spring, and the fail-safe spring is maintained in position by a solenoid, requiring around, but less than, 20 watts of power.
The patents noted herein provide information regarding the developments that have taken place in the field of fail-safe electric actuator technology. Clearly the instant invention provides many advantages over the prior art inventions noted above. Again it is noted that the invention, in comparison with prior art rotary fail-safe electric actuation, provides the following advantages:
higher torque/volume ratio;
improved simplicity of operation and maintenance
increased speed control; and
reduced power consumption to maintain fail-safe mode.
An actuator that provides increased torque/volume ratio, improved simplicity of maintenance and repair, increased speed control, and reduced power consumption to maintain fail-safe mode is disclosed. To increase the torque/volume ratio during normal and fail-safe operations, a novel clutching mechanism that enables three separate actuator modes is disclosed. To reduce complexity in maintaining and repairing the actuator, a dual motor system and the use of power springs are disclosed. One of the motors is used for sequencing operations, and the other for controlling actuator output. And finally, to reduce power consumption for maintaining fail-safe mode, and for maintaining torque during fail-safe operation, a power spring system, pawling mechanisms, and supporting clutching system are disclosed whereby energy is stored in power springs, maintained through pawling and worm gear systems, and released during clutch-controlled failure mode operation. Energy is maintained in the power springs not by a constant energy source but by gear locking mechanisms, which allow rotation in only one direction. When in fail-safe mode, the positioning or main power spring is released at a controlled rate through the interaction of the clutching system and an escapement mechanism.
The basic function of the disclosed actuator is to direct energy flow through three separate modes of operation: energy storage mode, run mode, and fail-safe mode. During power-up, the smaller of the two motors delivers the required amount of energy to the smaller of the two power springs to achieve desired tension in the spring so that spring can be used to drive the cams which set the actuator""s mode properly during fail-safe operation. This spring is later used during fail-safe operation and its stored energy is maintained through a worm gear locking mechanism. Also during power-up, the larger of the two motors delivers the required amount of energy to the larger of the two power springs, the main spring, the spring that drives the output during fail-safe operations. The stored energy in this spring is maintained through a specially-formed pawl which has a tooth positioned between gear teeth in one of the cam gears. Upon completion of energy storage, the actuator is placed into run mode. Once in run mode, the actuator is free to function normally. During a power or signal loss, the actuator is taken out of run mode and placed in fail-safe mode through the energy stored in the smaller of the two springs. This condition allows the main spring to discharge, driving the output to full clockwise or counterclockwise positioning. Each of these modes is discussed in the following paragraphs.
When power is supplied to the actuator, electronic circuitry determines the condition of the actuator. If the actuator is in low energy state, the secondary or sequencing motor drives the cam mechanism which sets the clutches properly, providing a path for energy to flow to the power springs. Energy storage continues until the tension required to rotate the main spring shaft overcomes a pre-load spring and trips a micro switch, cutting power to the motor. Energy storage is maintained as described above.
After the first micro switch has been tripped, the secondary or smaller motor continues to drive the cam mechanism in the same direction until the final position micro switch is tripped. The clutch mechanism is now providing a path for the larger motor to drive the output gear train of the actuator with the fail-safe springs fully wound and on stand-by. At this point, the actuator is operating normally.
During loss of signal or power interruption, the actuator circuitry applies reverse polarity across the solenoid, which causes the solenoid plunger to extend, with the aid of a small spring. The solenoid plunger/spring combination removes or disengages the worm from the worm wheel, which allows the smaller spring to discharge, thus forcing the cam/clutch mechanism to return to the initial low energy condition. In this position, after placing the motor gear train in a neutral position, a path is provided for the main power spring to discharge to the output of the actuator. As the spring is unwound, an escapement mechanism, which is located between the drive gear and the main spring, controls the rate of release of the energy. The escapement, a clockworks-type mechanism, rocks between the spring and drive gear, periodically engaging on its edge with the drive gear, which slows the rate of energy release from the main spring. The rate of release of the spring""s energy is selectable based upon the desired reverse time of the actuator.
The dual-cam subsystem is comprised of two slotted cams, two interlocked cam gears, and two bearing cup assemblies, among other parts. The cams are hollow cylinders housing bearing cup assemblies and cam pin drivers which are attached each to the interlocking cam gears. Pins, one for each cam, which are elongated, solid, thin cylinders, are slidably inserted through cam slots, bearing cup assemblies, and cam pin drivers. The slots spiral partially up the circumference of the cams. Thus, the rotation of the interlocked cam gears causes the pins to move within the slots, among other things, and the position of the pins within the slots relative to each other indicates the actuator""s mode.
Connected concentrically with the dual-cams are the dual-clutches comprised of clutch shafts, gears, pinions, and clutch bodies. Each clutch body is a disk which is slidably positioned on the clutch shaft between pinions and gears at each of the ends of the clutch shaft. Each pinion is also disk-shaped, and is fabricated with slots for accepting pins. On each face of each clutch body disk are positioned retractable is tapered pins which retract into the recesses in the clutch body upon pressure. Within the clutch body and adjacent to the base of each pin is a spring that lies between the two pins protruding from opposite sides of the clutch body disk. The pins are designed to retract only so far as the spring will allow, but far enough so that the pin will not inhibit the movement of the clutch body or pinion with respect to each other until the pin is anchored in a slot. During gear changes, the tapered pins become seated in the slots of either one of the slotted output pinions that surround the clutch assembly on the clutch assembly shaft. Because the output pinions are slotted, the tapered pins do not have to line up exactly before engaging. Also, the pins are tapered at such an angle as to reduce the force required to overcome the friction that occurs between the pins and the slots during gear changes.
Thus, the disclosed electric rotary actuator is comprised of novel components that accomplish fail-safe energy storage, controlled energy release during fail-safe operation, dual clutch and cam subsystems that provide for three separate modes of operation within a relatively small volume and with a relatively small power consumption.
It is therefore an object of the present invention to provide increased torque/volume ratio in both normal and fail-safe modes through a novel clutching mechanism, the use of power springs, and other novel design features.
Another object of the present invention is to provide an increase in reliability through reduction in complexity in operation and maintenance. Separation of functionality and control of various functions through dual motors provides decoupling of interactions and thus a reduction in the complexity of problem-solving and operational downtime. Unique design provides for some on-site maintenance, which also can reduce effective downtime.
A further object of the invention is to provide reduced power consumption required for maintaining fail-safe capability. Reduction in power consumption is accomplished through the improved way in which stored energy is maintained through pawl and worm gear mechanisms.
A further object of the invention is to provide increased speed control during positioning. This is accomplished through the novel escapement mechanism of the instant invention.
A yet still further object of the invention is to provide a design that can accommodate either DC or AC motors.
A yet still further object of the invention is to provide a design that can accommodate either a latching or non-latching solenoid.